Long Valley Caldera: The Supervolcano Under Your Feet
Near Mammoth Lakes, California
The scenery around Mammoth Lakes is beautiful, wide green valley, hot springs steaming in the cold, snowcapped peaks. What almost nobody tells you is that you are standing inside the scar of one of the largest volcanic eruptions in the history of North America. About seven hundred sixty thousand years ago, the ground here tore open and erupted.
Geologists estimate it threw out something like six hundred cubic kilometers of ash, pumice, and molten rock. That is more than two thousand times the volume of the nineteen eighty Mount Saint Helens eruption. Ash from this single event has been found as far away as Nebraska and Kansas.
When the magma chamber emptied, the roof of the volcano collapsed into the hole it left behind, forming a giant oval basin called the Long Valley Caldera, roughly ten miles by twenty miles across and once nearly three thousand feet deep. The rock that fell out of the sky and roared across the land in glowing avalanches cooled into a formation geologists named the Bishop Tuff, and you can still see its pink and tan cliffs in the gorge below Crowley Lake. Here is the part that keeps scientists watching.
This volcano is not dead. It is only sleeping. In the center of the caldera the ground has been slowly bulging upward, a feature called the resurgent dome, and since about nineteen eighty it has risen roughly two and a half feet, pushed from below by molten rock.
In May of nineteen eighty, over a single stretch of about two days, four magnitude six earthquakes struck here. The United States Geological Survey took notice and set up one of the most closely monitored volcanic observatories in the country. Today they track the ground with satellites, tilt meters, and hundreds of seismometers, measuring every breath the caldera takes.
The most recent eruptions in this system happened only a few centuries to a few thousand years ago, which by geologic standards is yesterday. So enjoy the hot springs and the mountain air. Just remember that the warmth rising out of the ground is the same fire that once buried half a continent in ash.
